Solid state sub-millimeter device utilizing cerenkov effect



Dec. 27, 1966 w. M. HONlG 3,2

SOLID STATE SUB-MILLIMETER DEVICE UTILIZING CERENKOV EFFECT Filed Feb. 16, 1965 INVENTOR. WILLIAM M. HONIG ATTORNEYS Patented Dec. 27, 1966 3,295,068 SGLID STATE SUB-MILLEMETER DEVICE UTILIZKNG CERENKQV EFFECT William M. Honig, 6801 Bay Parkway, Brooklyn, N.Y. 11204 Filed Feb. 16, 1965, Ser. No. 432,948 3 Claims. (Cl. 331-81) The present invention relates to oscillators, and, in particular, to an oscillator capable of producing submillimeter and microwave oscillation and operating by use of the phenomenon of Cerenkov radiation.

The techniques presently used for the production of microwave power are not generally suitable for the generation of practical amounts of energy in which the wavelength is approximately equal to or less than one millimeter. Accordingly, it is an object of the present invention to provide a practical method and apparatus for the production of millimeter and sub-millimeter microwave energy.

Another object of the invention is to provide a submillimeter microwave generator wherein the frequency may be varied.

Another object of the invention is to provide a practical apparatus and method for the utilization of the Cerenkov effect in the production of microwave energy.

Cerenkov radiation, as explained more fully below, is generally manifested as broad band electromagnetic radiation resulting when the speed of an electron stream exceeds the speed of the electromagnetic waves propagated by the electrons in a surrounding medium.

It is known that Cerenkov radiation may be used for the generation of radio frequency energy in a narrow wavelength range provided that the electrons are appropriately bunched. With prior art configurations, electron bunching was obtained by means of an externally generated and controlled alternating field in a conventional way which severely limited use of the Cerenkov efiect for the production of useful electro-magnetic radiation.

In the present invention, however, a stream of electrons is subjected to' the alternating field of a portion of the generated Cerenkov radiation so that certain electrons are accelerated and others are decelerated thereby forming bunches of electrons.

Accordingly, it is a further object of the invention to provide a microwave generator employing the Cerenkov effect wherein no externally generated bunching field is required.

Yet another object of the invention is to provide a solid state device capable of generating sub-millimeter microwaves.

The manner in which the above and other objects of the invention are accomplished is more fully described below with reference to the attached drawings wherein:

FIGURE 1 is an explanatory diagram of the Cerenkov effect;

FIG. 2 is a schematic illustration of a practical embodiment of the invention.

When an electron moves in a medium at a velocity having a certain relation to the speed of light in that medium, an asymmetric polarization field results which will cause the radiation of a brief electromagnetic pulse or wavelet at each point along the path of the electron.

For example, referring to FIGURE 1 the path of an electron is indicated by the line L with the electron moving in a direction indicated by the arrow 10. If three incremental points P1, P2, and P3, on the path L are examined, the electron, at each point, may be considered to radiate wavelets having wave fronts indicated by the arcs A1, A2, and A3, respectively. In the usual case, these radiated wavelets interfere destructively so that at a distant point the resultant field intensity is zero. However, if the velocity of the electron along path L is higher than the phase velocity of the light in the same medium, a resultant field can be observed from a distant point at the indicated angle 0 with respect to the track L.

The angle 6 is the angle at which the radiated wavelets from all of the arbitrary points such as P1, P2 and P3 are coherent and may be considered to combine to form a plane wavefront BC (according to Huygens principle). Such coherence occurs when the electron traverses the path L from A to B in the same time that the light travels from A to C.

Cerenkov showed that under the above conditions Cosine 0:1/ 811) which is known as the Cerenkov relation, wherein:

18 equals the velocity of the electron (v) divided by the velocity of light in a vacuum (0); and

n is the reflective index of the medium, which is equal to the square root of the product of the dielectric constant (e) and permeability (a) of the medium.

Since cosine 0 cannot be greater than one, there can be no Cerenkov radiation unless B G/n), or v m/ea.

A more detailed description of the Cerenkov eitect can be found in the book, Cerenkov Radiation and Its Application, by J. V. Jelley (195 8), Pergamon Press.

If the velocity of the electron (1 can be made to approach the velocity of light in a vacuum (c), the required product ea is readily obtainable. For such electron velocities to exist it is generally necessary to generate and accelerate the electron beam in a vacuum; then it is necessary to pass the beam in close proximity to the Cerenkov material so that the Cerenkov relationship holds. In such a case, a vacuum tube construction is required with all the attendant disadvantages of such tubes.

In attempting to radiate Cerenkov waves by the movement of electrons in a solid, a serious problem arises because of the relatively low velocity of electrons in a solid. In most cases, the velocity of the electron will be so low that the required values of e and p. are prohibitively high. However, by a suitable selection and arrangement of an appropriate conduction material and an appropriate dielectric material, it is possible to achieve the conditions necessary for the solid state generation of Cerenkov radiation.

For example, conduction electrons in indium antimonide at 77 K. reach drift velocities of 0.3% of 0 (free space electromagnetic velocity). These measurements have been made on bulk material with electrical fields of to 400 volts per centimeter. Moreover, the velocity of the electrons may be increased by a factor of 2 to 3 by means of an axial magnetic field. Such high velocities may be achieved in solid state plasmas by means of two stream instabilities and screw instabilities.

In accordance with the Cerenkov relation, for electrons traveling at a speed of between .3 and 1% of c, it is necessary that the product of e and a be between 90,000 and 10,000. This criterion can be met in selected materials.

The dielectric constant for the single crystal ferrimagnetic materials (yttrium iron garnet) are in the range of 15 to 30 and the permeability reaches values of at least 3500 at gyroma gnetic resonance. As a practical matter, these values occur in the 1-30 Kmc. region.

If the required product ea is 10,000, and the material has a dielectric constant of 30, its permeability must be greater than 333 which value can be achieved with antiferrimagnetic materials in the sub-millimeter region with gyromagnetic resonance due to exchange fields. On the other hand, a pure dielectric such as strontium titanate has a dielectric constant as high as 310 in the one millimeter region but the accompanying permeability of approximately 30 is apparently not available.

Apparatus for practically employing the principles of the invention set forth above is schematically illustrated in FIGURE 2. The conductive material is formed as a rod (or as a hollow cylinder), and may, for example, be indium antimonide or any solid material in which the conduction electrons are capable of obtaining relatively high velocities as described above. The conduction rod 10 is coated with a dielectric material 12 having a dielectric constant and permeability such that the Cerenkov relation will hold true as explained above. In particular, for the generation of sub-millimeter waves, the coating 12 may consist of anti-ferrimagnetic substances such as Cr O NiO, CrSb, MnF For the generation of millimeter and microwaves the dielectric material 12 may consist of ferrite garnet, yttrium iron garnet (YIG), GaYIG, or LiYIG.

A pair of ohmic contacts 14 and 16 are secured at opposite ends of the coated rod 10, and a source of direct voltage 18 connected across ohmic contacts 14 and 16 to produce an electric field for acceleration of the conduction electrons through rod 10. A conically shaped Cerenkov coupler indicated at 20 surrounds the coated rod 10 and reflects the Cerenkov radiation as a plane wave through a high Q resonant circuit 24 and to an optical type reflector 22. Cerenkov couplers per se are known, and may, for example, be made of titanium dioxide or Teflon. Instead of using a separate material for the coupler 20 it is possible that the material of the coating 12 be arranged to also serve for the coupler.

The high Q resonant structure 24 limits the broad band Cerenkov radiation to a narrow frequency range which is coupled to reflector 22. Reflector 22 can be a conventional dielectric and serves the important function of feeding a portion of the selected output frequency b ack to the Cerenkov wave generation region as a bunching field for the electrons within rod 10. The proper launching and phasing criteria are obtained in accordance with known practice. Thus, the electrons will be bunched without the necessity of an external field for this purpose, and the generation of the Cerenkov radiation will proceed with improved efficiency as noted above. Because the actual oscillator frequency is fed back to bunch the electrons, and the electron velocity is relatively low, there is no problem in obtaining bunches of electrons smaller in size than the wavelength of the oscillation.

The apparatus illustrated feeds energy back to the portion of the conduction material where the Cerenkov radiation originates for the purpose of electron bunching but this is not essential to practice the invention. For example, as an alternative the energy may be fed back to a portion of the oonductionmaterial provided in ad- Vance of the Cerenkov interaction region (not shown in the illustrated embodiment), in which case the bunching may precede the interaction, with an intervening drift space (analogous to a klystron) The resonant structure 24 may be a gap which is mechanically variable by positioning reflector 2'2. Alternatively, the resonant circuit 24 and reflector 22 may comprise a nulti-layer interference reflector in which the thickness of the layers determines the frequency of the reflected energy. The lines 26 indicate the path of the electromagnetic waves after they are generated within dielectric 12 and reflected by Cerenkov coupler 20 and reflector 22.

The output energy may be derived from a number of points in the illustrated system. By way of example only, an output probe is indicated schematically at 28 to draw off the output energy. One or more iirises may be used to provide an output port or an annular coupling slot may be employed.

While the invention has been described in terms of an oscillator or generator of electromagnetic radiation, it is obvious that it may readily be used as a tuned regenerative amplifier by reducing the gain below the threshold of oscillation and providing an input signal at the resonant frequency.

Although preferred embodiments of the invention have been illustrated and described, the invention is not so limited and should only be defined by the following claims.

What is claimed is:

1. A short wavelength radio frequency device comprising .a solid conduction material adapted to support high velocity conduction electrons, means for producing a flow of high velocity conduction electrons in said solids, a Cerenkov material having a high dielectric constantpermeability product, said Cerenkov material being situated in proximity to at least a portion of said conduction material to generate Cerenkov radiation in said Cerenkov material in response to the flow of electrons in said conduction material, a frequency determining structure adapted to select radiation at the operating frequency of said oscillator, a Cerenkov coupler arranged to couple the radiation from said Cerenkov material coherently to said frequency determining structure, means for supplying a portion of the energy selected by said frequency determining structure to a substantial portion of said conduction material to create a field for bunching the electrons flowing in said portion, and output means for coupling a portion of the energy selected by said frequency determining structure to external utilization apparatus.

2. A short Wavelength radio frequency device as claimed in claim 1, wherein said solid consists of indium antimonide.

3. A short wavelength radio frequency device according to claim 2, wherein said Cerenkov mate-rial is chosen from the group consisting of Cr O NiO, CrSb, MnF ferrite garnet, yttrium iron garnet, gallium yttrium iron garnet, and lithium yttrium iron garnet.

References Cited by the Examiner UNITED STATES PATENTS 3,178,656 4/1965 Petroff 331-79 FOREIGN PATENTS 782,573 9/1957 Great Britain.

Primary Examiner, 

1. A SHORT WAVELENGTH RADIO FREQUENCY DEVICE COMPRISING A SOLID CONDUCTION MATERIAL ADAPTED TO SUPPORT HIGH VELOCITY CONDUCTION ELECTRONS, MEANS FOR PRODUCING A FLOW OF HIGH VELOCITY CONDUCTION ELECTRONS IN SAID SOLIDS, A CERENKOV MATERIAL HAVING A HIGH DIELECTRIC CONSTANTPERMEABILITY PRODUCT, SAID CERENKOV MATERIAL BEING SITUATED IN PROXIMITY TO AT LEAST A PORTION OF SAID CONDUCTION MATERIAL TO GENERATE CERENKOV RADIATION IN SAID CERENKOV MATERIAL IN RESPONSE TO THE FLOW OF ELECTRONS IN SAID CONDUCTION MATERIAL, A FREQUENCY DETERMINING STRUCTURE ADAPTED TO SELECT RADIATION AT THE OPERATING FREQUENCY OF SAID OSCILLATOR, A CERENKOV COUPLER ARRANGED TO COUPLE THE RADIATION FROM SAID CERENKOV MATERIAL COHERENTLY TO SAID FREQUENCY DETERMINING STRUCTURE, MEANS FOR SUPPLYING A PORTION OF THE ENERGY SELECTED BY SAID FREQUENCY DETERMINING STRUCTURE TO A SUBSTANTIAL PORTION OF SAID CONDUCTION MATERIAL TO CREATE A FIELD FOR BUNCHING THE ELECTRONS FLOWING IN SAID PORTION, AND OUTPUT MEANS FOR COUPLING A PORTION OF THE ENERGY SELECTED BY SAID FREQUENCY DETERMINING STRUCTURE TO EXTERNAL UTILIZATION APPARATUS. 